Corrosion Fatigue Performance of Duplex 2507 for Riser Applications

Authors: Ramgopal Thodla, Feng Gui, Ken Evans, DNV Columbus; Carlos Joia, Ilson Palmieri Baptista, PETROBRAS/CENPES/PDP/TMEC Cidade Universitária

Source: CORROSION 2010, March 14 - 18, 2010, San Antonio, TX

Copyright 2010. NACE International

Keywords: Corrosion fatigue, Fatigue Crack Growth Rate, 2507 Duplex Stainless Steels

Preview ABSTRACT

This work investigated the corrosion fatigue performance of 2507 duplex stainless steel for use as riser materials in environments containing high partial pressures of carbon dioxide (50 - 100 bar) and limited quantity of hydrogen sulfide (0 - 0.12 bar). The procedures developed for controlling oxygen and Fe2+ contamination as well as methods to evaluate the concentration of H2S in the autoclave are presented. The crack growth rates and ΔKth for these materials in the pressure environments were discussed along with procedures to obtain ΔKth, when they were below 5ksi√in. Low crack growth rates in the range of 1e-8 in/cycle were measured and the effect of sour environments was quantified. The fatigue crack growth rate in sour environments on 2507 duplex stainless steel is a 10x higher than in air.

INTRODUCTION

Corrosion fatigue of C-Mn steels has been extensively studied for riser applications in sour service environments1-4. Fatigue crack growth rate (FCGR) work on high strength steels performed in sour service showed that there was an increase in fatigue crack growth ranging from 3x to 100x depending on the environmental conditions1-4. Frequency effects on corrosion fatigue have also been extensively studied and it was found that decreasing frequency led to an increase in the FCGR of C-Mn steels in sour environments.

The fatigue performance of welded joints in duplex stainless steels in air is broadly similar to that of C-Mn structural steels1-3. FCGR in sea water in the high ΔK regime was (700-1000N/mm- 3/2 air1-3. The effect of cathodic protection would enhance this effect. This is consistent with the hypothesis that FGCR is controlled by hydrogen embrittlement with crack propagation through the ferrite phase4-5. It was found that corrosion fatigue in seawater in the HAZ, which had a higher ferrite content, showed higher propagation rates than parent steel4-5.

However, there has been very limited data on understanding the corrosion fatigue behavior of duplex stainless steels in sour environments relevant to oil and gas production. A limited amount of work suggests that in the presence of sour environments, the FCGR is enhanced by about 10x over the values of air6. There have been some studies on the effect of key environmental variables such as pH, H2S concentration and temperature.   However, there has been very little work performed on duplex steels which are candidate materials for risers in deep water containing sour environments. The focus of the current work was to develop procedures to measure FCGR in high pressure sour brine environments that are relevant to the oil and gas industry. The work was focused on developing controls on the environmental test parameters as well as measuring FCGR at very low ΔK values.

EXPERIMENTAL

Two sets of experimental measurements were performed in this test program. The first set of measurements were electrochemical measurements on a Super 13Cr steel (SM13CRS-110*) with YS of 110ksi to identify a suitable reference electrode as well as estimate.) was significantly higher than in air.

Number of Pages 15